An internal reinforcement structure of a plastic fuel tank resists deformation of opposing walls of the fuel tank and provides an integral, and directionally sensitive, stress relief feature when pre-determined forces are exceeded. The stress relief feature is contained within a fuel chamber of the fuel tank defined by the opposing walls. Each wall has an inward projecting indentation of the structure which engage one-another at their distal ends or bottom portions, preferably, via a welded plastic engagement area. The indentations have a consistent wall thickness which has a higher cross-sectional area than the stress relief feature causing the stress relief feature to tear as opposed to the tank walls thereby assuring fuel tank integrity.